1. Field of the Art
The present invention relates to a conductive tin oxide sol and a process for producing the same, and further a coating composition by use of the conductive tin oxide sol and a covering material by use of the coating composition.
2. Description of the Related Art
As tin oxide having electrical conductivity, tin oxide containing antimony, that is, antimony-doped conductive tin oxide (ATO) is known. Although ATO is excellent in electrical conductivity, recently the toxicity of antimony and intense coloration (blue-black) resulting from antimony become problems, and therefore conductive materials containing no antimony are desired. Thus, several conductive tin oxides in which alternative elements such as phosphorus, fluorine, or the like is doped instead of antimony are proposed.
Conductive tin oxide containing no antimony is produced by for example any of the following methods:
a method in which a liquid prepared by dissolving germanium, phosphorus, lithium and zinc in a tin chloride solution is added in an alkaline aqueous solution to form a precipitate, and the precipitate is calcined at 350 to 700° C. (see, JP-B 2-32213 (1990));
a method in which a soluble phosphorus compound is added in a hydrous tin oxide precipitate prepared by reacting an acid or an alkali with a tin compound solution to form a precipitate, and the precipitate is calcined at 800 to 1300° C. (see, JP-A 6-92636 (1994)); and
a method in which an aqueous solution of a tin compound is subjected to hydrolysis of the tin compound by keeping the solution at pH 8 to 12 to form a sol containing colloidal particles of a metal oxide and/or a hydrous oxide, the colloidal particles are recovered from the sol, and then at least one aqueous solution of an antimony compound, a phosphorus compound and a fluorine compound is impregnated therein, dried and calcined (see, JP-A 63-11519 (1988)).
In these methods, as powders are obtained by calcining at a high temperature of 350 to 1300° C. in order to acquire electric conductivity, the dispersion properties thereof are not good, and thereby these powders are finely divided to a particle size not less than about 0.05 to 0.4 micrometer even by using a normal grinding means.
In recent years, in order to make transparent plastics antistatic, antistatic coatings with a high transparency that does not injure the transparency of substrates are required, but the coating films by use of the above-mentioned powders cannot provide a high transparency.
In addition, a sol of conductive tin oxide is prepared by heating tin oxide powders that are doped with a small amount of alternative elements such as antimony, fluorine, phosphorus, tellurium, bismuth, cadmium, or the like in an acid aqueous solution or an alkaline aqueous solution at 30 to 200° C. (see, JP-A 62-230617 (1987)).
However, as the calcination at a temperature of 350° C. or more in the process of doping alternative elements is carried out, the average particle diameter of dispersed particles contained in the obtained sol is not less than about 0.03 to 0.10 micrometer. This process cannot provide highly transparent coating films.